This invention relates to an articulated rotor head for gyroplanes, comprising a rigid central hub body which is perpendicularly integrated into a drive shaft. Each blade of this rotor head is fixed radially to the hub body by means of connecting and articulating components. In accordance with the invention, the rotor head, articulated in pitch, beating and drag, is intended particularly for an helicopter.
Many articulated rotor heads for gyroplanes are already known, especially those developed by the Applicant and filed under French patents numbers 2 427 251, 2 456 034, 2 516 891, 2 584 995 and 2 584 996. While this type of hub achieved great simplicity of design, especially with respect to its integrated mast-hub shape, it does however include a number of sensitive parts. Indeed, a spherical laminated elastomer stop provides by itself, in the presence of the centrifugal forces generated by the blade and which subject it to compression, the beating, drag and pitch control articulation functions of the blade. Thus the spherical laminated elastomer stop is one of the main elements in the definition of the hub and governs its dimensioning. Another sensitive point concerning this type of rotor head is the presence, for each rotor, of an elastic return brace with incorporated damper which controls the angular movements of the blade within its plane and ensures the stability of the rotor when it encounters phenomena of ground and air resonance. This elastic return brace with incorporated damper follows the beating, drag, and pitch control movements of the coupling sleeve of the corresponding blade. This results in pitch-drag couplings which in particular generate increased forces on the controls. Moreover, this brings about large angular movements on the trunnions on the ends of the elastic return brace with incorporated damper that are detrimental to the long service life of these components, and which also have to withstand heavy static and dynamic radial forces.
For the working conditions of the present generation of helicopters, these problems can be solved relatively easily using these types of rotor which then provide suitable in-service strength. Nevertheless, rotary wing aircraft development over the next few years will certainly result in enhanced in-flight performance, particularly with respect to speed, subjecting rotor hubs to even greater stress and strain. In particular, variations in collective pitch and cyclical pitch will increase significantly, and problems to do with the endurance strength of spherical laminated stop and the trunnions of the elastic return and drag damper braces of the blade will become more acute.
To meet these requirements, solutions have been proposed for articulated rotors based on a combination of several types of thrust stop for each blade, as for example contained in European patent 150 331 791 and in U.S. Pat. No. 4,203,708.
European patent 0 331 791 relates to a structure in which the hub body is connected to the top of the mast by means of a single central spherical laminated trunnion. For each blade, the hub body carries a spindle integrated with the hub body, on which is mounted a connecting sleeve to the blade. This sleeve is mounted on the spindle by means of two bearings and a laminated thrust stop in the shape of a flat crown interposed between the spindle and the sleeve. In this patent, the central spherical laminated trunnion allows limited angular oscillations about the center of this trunnion, thus reducing in this so-called "rigid" type of rotor the anchoring moment of the hub on the mast. This type of single trunnion therefore allows only trimming of the whole rotor, but has the drawback of never permitting individual beating, drag and pitch angular blade movements.
Moreover, this central laminated trunnion is not affected by the centrifugal forces generated by the blades, which balance out from one blade to the others through the outer armature of this central trunnion. In this type of assembly, the pitch control is mounted on the sleeve which is outside the spindle. In this arrangement, the outside sleeve is acted upon by the pitch control, but does not receive any beating and drag movements, which are ensured by the flexibility of the blade neck. The drawbacks mentioned above concerning pitch control via the sleeve are encountered once again. In fact, the main objective of the invention described in this document would appear to be the embodiment of the hub, in which the combination of two laminated stops provides a seal against oil leakage.
U.S. Pat. No. 4,203,708 relates to an articulated rotor head in which each blade is fixed to the hub body by means of two laminated thrust stops: one spherical and one flat. In such embodiment, a connecting sleeve is arranged between the blade and the set of stops, the pitch control being mounted directly on this connecting sleeve. Thus in this type of assembly, the two thrust stops are coupled together and work in series on the same spindle which is arranged at the inner end of the connecting sleeve. For this reason, this structure does not separate the beating and pitch control functions into two different parts and therefore subjects the damper, the inner end of which is connected to the hub body, and outer end to the foot of the blade, to large drag-pitch forces. Indeed, the pitch changes cause the damper coupling point next to the blade to turn in the arc of a circle and consequently lead to variations in the distance between the damper fastening points on the blade and hub sides. This results in dynamic parasitic movements of the damper, as well as large movements of the trunnions located at the fastening points of this damper. In addition, this hub arrangement makes it difficult to provide bottom stops that allow the blade to be maneuvered in pitch over all the permitted range when the rotors are stopped and the blades resting on their stops, in order to test the free movement of flight controls before starting to rotate the rotors, as required by flight safety rules. In this case, in fact, each blade-carrying spindle turns about its longitudinal axis and the bottom stop device must permit this free rotation, while the contact point on them is heavily loaded by the resumption of the momentum of the weight of the blade, whose center of gravity is moved several meters further away. Finally the two laminated thrust stops in this structure are arranged inside the hub body, which means that it is not easy to determine their state for inspections without first dismantling the assembly.